That depends on the locomotive. Some were (and are) equipped with six axle trucks with only four motors, but in recent (since mid-1960s) practice, the vast majority of six axle locomotives have come with six motors. Passenger engines by almost all builders from the 1930s to 1960s had "A1A" trucks with the middle axle unpowered. Only in the last few years have unpowered axles returned, on freight locomotives intended for fast operation. Some freight engines built in the 1940s and 1950s for operation on very light rail also had a center idler axle.

With the advent of 'inexpensive' AC traction motors and inverters, several railroads are buying or building "A-1-A" versions/conversions of six-motor locomotives, usually with some sort of traction-increasing device that unloads the idle axle for starting or periods when higher TE with limited wheelslip is desirable. (Not all these conversions put the idler as the center axle, but in my opinion the successful ones do.) There is some argument that four AC axles don't provide some of the advantages six DC axles do; on the other hand, most of the applications that would stress an A-1-A high-horsepower AC unit would involve six-motor AC and not legacy DC transmission.

There are also provisions on some locomotives to cut power to one or more TMs ("powered axles") in a given consist. This is different from physical electrical cutouts (of the sort that PC said not to use!) which for electrical cabling reasons often go in 'pairs.'

Why would you equip a locomotive with six motors and not power all of them in normal operation?

We've covered a number of reasons for this in recent posts.

The most immediate reason is when a company policy, or output from a computerized train-management system, indicates that the number of powered axles desired, or permitted, for a given consist in a particular situation is less than the full number of motored axles on the active units. We had a recent example (I believe somewhere in a discussion of stringlining) where 23 axles were indicated; on some locomotives (GEs) the TM in question is isolated via the locomotive computer.

Obviously, if something goes wrong with a traction motor in service that does not lock the motor pinion, you would want to isolate that motor (and any other through which it is connected) to continue making at least some power from the affected locomotive.

I believe there are some locomotives in preservation that operate on one of two prime movers (E units and 44-tonners being two that come to mind as example types) and it may make more sense operationally to route the power through just one truck. Naturally you wouldn't want to 'disturb the historic fabric' by removing the temporarily-unused motors from the other truck...

Why would [Baldwin] equip a locomotive with six motors and not power all of them in normal operation?

I guess in normal operation the six motors were powered. Don't recall what explanation the Railway Age (or maybe RME) article gave for the switch that could cut two motors out, but I don't think it was anything like the limit-maximum-TE switch that some UP units have now.

Is it possible that he is talking about the Essl modular Baldwin design that used genset-style engines with each driving its own dedicated motor?

The full 6000hp version of this would have had eight engines with eight motors, and you would tailor the horsepower by firing up an appropriate number of prime movers and energizing only the corresponding traction motors. Much of the time you would only be running on some smaller number of engines, and of course you wouldn't remove the unneeded motors from the frame when that was so.

I do not know if there was any attempt to modify the electrical gear to allow some combination of engines to work with, say, groups of four motors with series/parallel transition, which would get around some of the potential slipperiness of running 750hp through an individual unconjugated axle with prewar slip control. In any event the thing was too complicated, and by implication expensive, to produce in competition with other types of locomotive.

The arrangement is so that an AS-616 can match the 'speed-load curve of a four-motor unit' (presumably an AS-416) while retaining the use of six motors for slower, heavy service.

It would stand to reason that Railway Mechanical Engineer would have a more detailed account of the arrangement when it was announced ... this gives a timeframe for checking there. I wouldn't be surprised to find that Will Davis has a more thorough technical discussion from Baldwin in some of his source material (the merger with Lima-Hamilton had not come yet).

Does someone know if on a six traction motor on a locomotives powered or just four

I suspect what you're really asking is, if a diesel has six axles, are all six axles powered (equipped with motors) or just four?

The answer as has been touched on in earlier replies is "it depends".

Some diesels with six axles used A-1-A / A-1-A trucks, meaning the outside axles had motors and the middle axle was unpowered - no motor. This was common on passenger diesels, but some freight diesels used A-1-A trucks to distibute the weight better on light track (like on a branch line). However most six axle diesels had C / C trucks, meaning all the axles had motors and were powered.

Most non-EMD six axle locomotives built before the mid-1970s had uneven axle spacing on each truck. EMD's three motor flexicoils had even axle spacing. A few GMD-1s built for Canadian service had A1A flexicoils, and the FL9's rear truck was A1A. GE switched to a different truck design with the U30C around 1968 with evenly spaced axles, but no A1As used it.

Most non-EMD six axle locomotives built before the mid-1970s had uneven axle spacing on each truck.

You might want to go into the technical reasons behind axle spacing.

A-1-A passenger trucks want a relatively low polar moment of inertia, so they have the traction motors as far 'inboard' as possible (only a negligible part of the gearcase is outside the truck wheelbase).

A C truck, when used in heavy freight service, has a different "optimization problem". If you want to keep the motors inside the wheelbase, two of them will have to 'face' each other, which is why Trimounts and some other designs (including some GSC cast trucks on Baldwins) have asymmetrical spacing.

On the other hand, weight transfer (and in extreme cases even truck sideframe vertical alignment) is affected by the direction in which the motor powers its adjacent axle through its bull gear. In the Trimount arrangement at least one axle will be moved in the opposite direction to the others. It's relatively easy to line up the three motors so they all turn the same way ... but then one of them will be hanging off the end of the truck structure, either reducing the potential fuel tank capacity (if inboard) or exposing the TM case to road insults (if outboard), and in either case taking the polar moment and various kinds of 'inertia' load way up.